This invention relates generally to gas turbine engines and, in particular, to a compliant shim used between the dovetail root base of a fan or compressor blade and the corresponding dovetail groove in a fan or compressor disk, together with a seal layer to seal a gap that exists between adjacent compressor blade platform elements.
As discussed in the Herzner et al., U.S. Pat. No. 5,160,243, when two pieces of material rub or slide against each other in a repetitive manner, the resulting frictional forces may damage the materials through the generation of heat or through a variety of fatigue processes generally termed fretting. Some materials, such as titanium contacting titanium, are particularly susceptible to such damage. When two pieces of titanium are rubbed against each other with an applied normal force, the pieces can exhibit a type of surface damage called galling after as little as a hundred cycles. The galling increases with the number of cycles and can eventually lead to failure of either or both pieces by fatigue.
The use of titanium parts that can potentially rub against each other occurs in several aerospace applications. Titanium alloys are used in aircraft and aircraft engines because of their good strength, low density, and favorable environmental properties at low and moderate temperatures. If a particular design requires titanium pieces to rub against each other, the type of fatigue damage just outlined may occur.
In one type of aircraft engine design, a titanium compressor disk also referred to as a rotor or fan disk has an array of dovetail slots in its outer periphery. The dovetail base of a titanium compressor blade or fan blade fits into each dovetail slot of the disk. When the disk is at rest, the dovetail of the blade is retained within the slot. When the engine is operating, centrifugal force induces the blade to move radially outward. The sides of the blade dovetail slide against the sloping sides of the dovetail slot of the disk, producing relative motion between the blade and the rotor disk.
This sliding movement occurs between the disk and blade titanium pieces during transient operating conditions such as engine startup, power-up or takeoff, power-down and shutdown. With repeated cycles of operation, the sliding movement can affect surface topography and lead to a reduction in fatigue capability of the mating titanium pieces. During such operating conditions, normal and sliding forces exerted on the rotor in the vicinity of the dovetail slot can lead to galling, followed by the initiation and propagation of fatigue cracks in the disk. It is difficult to predict crack initiation or extent of damage as the number of engine cycles increases. Engine operators, such as the airlines, must therefore inspect the insides of the rotor dovetail slots frequently, which is a highly laborious process.
Various techniques have been tried to avoid or reduce the damage produced by the frictional movement between the titanium blade dovetail and the dovetail slot of the titanium rotor disk. One technique is to coat the contacting regions of the titanium pieces with a metallic alloy to protect the titanium parts from galling. The sliding contact between the two coated contacting regions is lubricated with a solid dry film lubricant containing primarily molybdenum disulfide to further reduce friction.
While this approach can be effective in reducing the incidence of fretting or fatigue damage in rotor/blade pieces, the service life of the coating has been shown to vary considerably. Furthermore, the process for applying the metallic alloy to the disk and the blade pieces has been shown to be capable of reducing the fatigue capability of the coated pieces. There exists a continuing need for an improved approach to reducing such damage and assure component integrity. Such an approach would desirably avoid a major redesign of the rotor and blades, which have been optimized over a period of years, while increasing the life of the titanium components and the time between required inspections. The present invention fulfills this need, and further provides related advantages.
U.S. Pat. Nos. 5,160,243 and 5,240,375 disclose a variety of single layer and multi-layer shims designed for mounting between the root of a titanium blade and its corresponding groove in a titanium rotor. The simplest of these shims is a U-shaped shim designed to be slid over the root of the fan blade (see FIG. 3 of the ""243 patent). A disadvantage to this type of shim is that it has a tendency to come lose during engine operation. Also, it does not entirely eliminate the fretting between the groove and the fan blade root.
Various methods for sealing the gap formed between the adjacent edges of the platforms of installed fan blades are known in the art. Examples include U.S. Pat. Nos. 5,827,047; 6,146,099; and 4,183,720. The ""047 patent is typical of seals which are positioned under the platform of fan blades by means of special structural elements formed in portions of the fan blade. Such applications require significant changes to the existing structure of fan assemblies.
The ""099 and ""720 patents represent examples of the bonding of strips of material to the underside of the platforms of fan blades. While this appears to be a simple solution to the gap sealing problem, the method introduces problems with types, strength and durability of the bonding substance.
As can be seen, there is a need for an improved compliant shim to inhibit fretting between titanium components and a mechanism for holding such a shim in place during engine operation, as well as a need for a shim to seal the gap that exists between adjacent compressor blade platform elements.
The present invention uses an easily installed compliant shim element to position a seal element by means of an upstanding wall element. The shim element is easy to install and retains the wall and seal in proper position to seal the gap between adjacent fan blades. The centrifugal load of the rotating fan assembly forces the seal element firmly against the fan blade platforms. This structure supplies a simple solution for two complex problems of performance of turbine fan assemblies. The part is simple to manufacture in that it may be a sheet metal stamping.
An improved compliant shim for eliminating fretting between titanium components and a mechanism for holding such a shim in place during engine operation in accordance with the present invention comprises a compliant shim for use between the root base of a gas turbine fan blade and a dovetail groove in a gas turbine rotor disk to reduce fretting therebetween. The compliant shim has first and second slots for engaging tabs extending from the fan blade root. The slots and tabs cooperate to hold the shim during engine operation. An oxidation layer covers the compliant shim and reduces fretting between the blade and the compliant layer. The invention further comprises an extended seal layer element to seal the gap that exists between adjacent fan blade platform elements.
These and other features, aspects and advantages of the present invention will become better understood with reference to the following drawings, description and claims.